1. Technical Field
The present invention relates to a method for starting a brushless DC spindle motor in a hard disk drive memory subsystem, and more particularly to starting the spindle motor in the presence of varying ambient temperatures.
2. Related Art
A hard disk drive typically includes one or more rotatable storage media, or disks upon which data is encoded. The disks are mounted on the shaft of a spindle motor for rotation. Data is encoded on the rotating disks as bits of information using magnetic field reversals grouped in tracks. A transducer head supported by an actuator arm is used to read data from or write data to the disks. A voice control motor (VCM) attached to the actuator arm controls positioning of the actuator, and thus the transducer head position over a disk. Servo position data read from the disk is processed by the processor, enabling the processor to provide servo control signals to the VCM for proper positioning of a transducer head relative to a disk.
Temperature changes during startup can significantly affect components of the disk drive. The operating temperature of a drive can be up to 50 degrees Celsius higher than the drive when it first starts at room temperature. In particular, low temperatures at startup of the drive will significantly affect run-up of the spindle motor. Oil bearing spindles, or other hydrodynamic spindle motors, have increased drag torque at low temperatures, primarily due to increased viscosity of the bearing fluid. The increased drag is most significant for hydrodynamic bearing spindles, which may be used in very high speed hard disk drives, such as drives that operate at 10,000 revolutions per minute (rpm).
A specific disadvantage is that the increased drag of the spindle bearing at low temperature start conditions may be so high that the spindle does not come up to speed in a desired time. For a 10,000 rpm, hydrodynamic bearing spindle, the difference in power due to additional bearing drag can be 1.0 Watt between 10° C. and 25° C. This corresponds to a significant difference in drag torque. A spindle motor with a torque constant based upon this worst case start condition may significantly compromise the motor at nominal operating conditions. The increased drag at low temperature starts would demand a lower torque constant to meet necessary voltage headroom conditions. A typical design practice is to reduce the motor torque constant just enough to allow spindle motor spin-up within a desired time, while allowing sufficient voltage headroom for adequate speed control.
The spindle motor bearings in typical disk drive spindle motors are located very close to the motor windings. The motor windings are the most significant source of heat in a motor. Thus, as the motor starts, the bearings heat up and increase in temperature. The bearing drag torque is a function primarily of the viscosity of the bearing grease base oil and of the stiffness of the grease, both of which reduce with increased temperature. So the bearing drag torque reduces with time shortly after the motor starts spinning due to the heating of the bearings. By allowing more time to spin up to speed before closed loop motor control takes over, the bearing drag can be reduced.
An error checking procedure for the proper operation of disk drives, primarily magnetic disk drives, is to conduct a “time out test” at the start-up of the disk drive. In a time out test, if the disk drive does not reach full operational speed within the time out or specified period, it is deemed an error. Often, a spindle motor controller sends an error signal if the spindle motor cannot come up to speed during the time out period and the drive is turned off.